Paper lateral edge detector for printer

ABSTRACT

First and second light receiving windows are formed on a head cover of a thermal head. The first light receiving windows are disposed to cross the lateral edges of the first king-sized recording paper. The second light receiving windows are disposed to cross the lateral edges of the second large-sized recording paper. First and second phototransistors are provided on the thermal head so as to measure quantity of light passed through the first and second light receiving windows. When printing the first recording paper, the first light receiving windows are partially shielded with lateral edges of the first recording paper and the second light receiving windows are completely shielded with the recording paper. Positions of the lateral edges are determined from the difference between output signals of the two phototransistors. When printing the second recording paper, the first light receiving windows are shielded by light-tight shielding stickers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a paper lateral edge detector,and more particularly, relates to a printer for forming a full-widthprint.

[0003] 2. Explanation of the Prior Art

[0004] Recently, a digital camera is widely available. A user uses animage taken by the digital camera for a print by a color printer,additionally, for an observation on such as a personal computer. In thecolor printer, it is desirable to form a print without a marginsurrounding the image, so-called a full-width print or non-margin print.

[0005] There are various types of color printers, such as an ink jettype, a thermal printing type, and a heat transfer type. For example, ina color thermal printer, a color thermal recording paper (hereinafterreferred to as only a recording paper) having a yellow thermal coloringlayer, a magenta thermal coloring layer, and a cyan thermal coloringlayer is used. A light emitting element array on a thermal head ispressed to the recording paper during transport there of, and threecolors of the thermal coloring layers are developed in sequence, to forma full color image on the recording paper.

[0006] Considering positional deviation and skewing in a widthwisedirection of the recording paper when forming a non-margin print by thecolor thermal printer, the light emitting element array which is largerthan recording paper in width is used. Moreover, when heat emittingelements not in contact with the recording paper are driven, wasteheating occurs, so that it shortens useful life of the heat emittingelements. JPA No.9-272217 discloses a color thermal printer, in whichthe recording paper is prevented from skewing, to transport therecording paper in a straight line. Moreover, the lateral edge of therecording paper is detected by a CCD line sensor, to prevent wasteheating of the heat emitting elements not in contact with the recordingpaper.

[0007] The aforementioned color thermal printer disposes the CCD linesensor upstream of a thermal head. The thermal head is extended in a subscan direction, that is also a transporting direction of the recordingpaper. Accordingly, the lateral edge of the recording paper is detectedat a considerable distance from a recording position where the heatemitting element array and the recording paper contact. In skewed mannerof the recording paper, there is no coincidence in the lateral edgesbetween a detecting position detected by the CCD line sensor and therecording position of the thermal head. Accordingly, in a non-marginprint, there occurs a white stripe in the vicinity of the lateral edgeand waste heating of the heat emitting elements. In a margin print,width thereof is not rendered uniform.

[0008] It is possible to principally solve the problem by disposing theCCD line sensor as near the heat emitting elements as possible. However,it is difficult to dispose the CCD line sensor very near the heatemitting elements since both the CCD line sensor and the thermal headhave a size to some extent. If the CCD line sensor is disposed near theheat emitting elements, it causes a problem that output signal from theCCD line sensor is likely to become unstable according to a heatingstate of the heat emitting element array. Moreover, the CCD line sensoris relatively costly, thereby to increase manufacturing cost of theprinter.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a paper lateraledge detector, which can be disposed near a recording position.

[0010] Another object of the present invention is to provide a lateraledge detector for a recording paper, with a simple structure, whichenables detection of a high precision.

[0011] Still another object of the present invention is to provide alateral edge detector for a recording paper, which prevents heat damageby a thermal head.

[0012] To attain the above objects, the paper lateral edge detector ofthe present invention is provided with first and second photo sensors.The first photo sensor measures quantity of light passing through alight-receiving window partially shielded by the lateral edge of therecording paper. The second photo sensor is disposed away from the firstphoto sensor in the widthwise direction of the recording paper. Thesecond photo sensor measures quantity of light in a light-shieldedmanner, to output dark current. A judging means judges the position ofthe lateral edge of the recording paper from the difference signalbetween output signals of the first and second photo sensors.

[0013] The first and second photo sensors are attached to a recordinghead. A paper guide is mounted to the recording head for guiding therecording paper, on which the light-receiving window is formed.

[0014] According to the preferred embodiment of the present invention,the printer enables to print first and second recording papers. Thefirst recording paper is larger than the second recording paper inwidth. The first photo sensor measures quantity of light passing throughthe first light-receiving window disposed to cross the lateral edge ofthe first recording paper. The second photo sensor measures quantity oflight passing through the second light-receiving window disposed tocross the lateral edge of the second recording paper. The recording headis driven for printing on either the first or second recording papers ona line-by-line basis in a widthwise direction of the recording paper.The recording head is larger than the first recording paper in width. Ashielding means shields the second light-receiving window when printingthe first recording paper, and shields the first light-receiving windowwhen printing the second recording paper. A thermal printer includes athermal head on which a plurality of heat emitting elements is aligned.

[0015] According to the present invention, the paper lateral edge isdetected by the photo sensors, thereby to reduce cost. Moreover, thephoto sensor can be attached to the recording head due to its smallsize, so that it enables to detect the paper lateral edge near therecording position. Furthermore, the lateral edge of the recording paperis found from the difference signals between the first and second photosensors by using the second photo sensor which detects dark current inaddition to the first photo sensor which detects the paper lateral edge,thereby to prevent damage by thermal drift.

[0016] According to the present invention, a white stripe on the lateraledge is prevented in a full-width print or a non-margin print and thewidth of the margin is rendered uniform in a margin print since thepaper lateral edge is correctly measured. Moreover, if there occurspositional deviation of the recording paper in a main scan direction, itis possible to print an image on the recording paper in a manner thatthe center of the image is made to coincide with that of the recordingpaper in a main scan direction. Therefore, the incomplete printing ofthe image can be prevented, resulting in improved printing quality.Furthermore, the heat emitting elements can be protected from shorteningits useful life since it is possible to prevent waste heating of therecording paper.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other objects and advantages of the presentinvention will become apparent from the following detailed descriptionof the preferred embodiments when read in association with theaccompanying drawings, which are given by way of illustration only andthus are not limiting the present invention. In the drawings, likereference numerals designate like or corresponding parts throughout theseveral views, and wherein:

[0018]FIG. 1 is a schematic diagram illustrating a color thermal printerof the present invention;

[0019]FIG. 2 is an explanatory view illustrating a position of a heatemitting element array;

[0020]FIG. 3 is a plane view illustrating a thermal head and theperipheral thereof;

[0021]FIG. 4 is a side view illustrating a thermal head and theperipheral thereof;

[0022]FIG. 5 is a schematic circuit diagram illustrating a paper lateraledge detector of the present invention;

[0023]FIG. 6 is a block diagram illustrating an electrical configurationof the color thermal printer;

[0024]FIG. 7 is a flow chart illustrating printing operation;

[0025]FIG. 8 is an explanatory view illustrating a manner that arecording paper is deviated from the correct position during transportthereof;

[0026]FIG. 9 is a cross sectional view illustrating a manner that afirst light receiving window is shielded for printing on a recordingpaper having a narrow width;

[0027]FIG. 10A is a plan view illustrating a manner that a shieldingshutter covers a first light receiving window;

[0028]FIG. 10B is a plan view illustrating a manner that the shieldingshutter uncovers the first light receiving window; and

[0029]FIG. 11 is a perspective view of a main part illustrating anembodiment that two mask plates selectively covers the first and secondlight receiving windows.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] In FIG. 1, a color thermal printer is loaded with a roll 11 oflong thermal recording paper 10 being wound. The roll 11 is rotated by asupply roller 12 abutting on the perimeter of the roll 11, to feed orrewind the recording paper 10.

[0031] It is well known that the recording paper 10 includes a cyanthermal coloring layer, a magenta thermal coloring layer, and a yellowthermal coloring layer, overlaid on a support in sequence on oneanother. The yellow thermal coloring layer as a topmost layer, which ishighest in heat sensitivity among three thermal coloring layers, iscolored yellow with small heat energy. The cyan thermal coloring layeras a lowermost layer, which is lowest in heat sensitivity among threethermal coloring layers, is colored cyan with large heat energy. Theyellow thermal coloring layer loses its coloring ability when nearultraviolet rays are irradiated in a wavelength region in which the peakvalue is 420 nm. The magenta thermal coloring layer is colored withmedium-heat energy between the yellow and cyan thermal coloring layersand loses its coloring ability when near ultraviolet rays are irradiatedin a wavelength region in which the peak value is 365 nm.

[0032] A carrying roller couple 15 is disposed downstream of the roll 11in a feeding direction, for pinching and transporting the recordingpaper 10. The carrying roller couple 15 consists of a capstan roller 17and a pinch roller 18.

[0033] The capstan roller 17 is rotated by a transporting motor 16. Thepinch roller 18 is shiftable between the pressed position on the capstanroller 17 and the separated position from the capstan roller 17. Therecording paper 10 is reciprocally transported between feeding andrewinding (printing) directions by the carrying roller couple 15.

[0034] A thermal head 20 and a platen roller 21 are disposed downstreamof the carrying roller couple 15 in a feeding direction, to pinch atransporting path of the recording paper 10. The thermal head 20includes a thermal head substrate 22 formed of metal having high thermalconductivity, to which a ceramic plate (not shown) is attached. A heatemitting element array 23 and a head driver 67 (shown in FIG. 6) areformed on the ceramic plate. In FIG. 2, the heat emitting element array23 has many heat emitting elements 24 which are aligned along a mainscan direction perpendicular to a transporting direction (sub scandirection) of the recording paper 10. The heat emitting element array 23has enough length to cover the width of the recording paper 10 sinceprinting is performed on a whole range of the recording paper 10.

[0035] The platen roller 21, slidable in a vertical direction, is biasedby a spring (not shown) in a direction of being pressed on the thermalhead 20. Each of the heat emitting elements 24 is heated depending upona thermal coloring layer and the image data to be recorded, to print oneof three primary colors on the recording paper on a line-by-line basis.The platen roller 21 rotates by following the transport of the recordingpaper 10.

[0036] An end-detecting sensor 25 is disposed between the carryingroller couple 15 and the platen roller 21, for detecting the distal endof the recording paper 10 during paper transport. The end-detectingsensor 25 is, for example, a photo interrupter which includes aprojection area for irradiating inspection light to the distal end ofthe recording paper 10 and a receiving area for receiving inspectionlight reflected to the recording paper 10.

[0037] In FIGS. 2 to 4, photo sensors 27 and 28 (hereinafter referred toas paper lateral edge detecting sensors) are provided upstream of theheat emitting element array 23 in an rewinding direction, also near theheat emitting element array 23, for detecting both lateral edges of therecording paper 10. The paper lateral edge detecting sensors 27, 28 aredisposed asymmetric with respect to a centerline (CL) which shows acentral position of the transporting path in a main scan direction.

[0038] In FIG. 5, the paper lateral edge detecting sensor 27 as a firstsensor consists of light emitting diodes (LED) 30,31 as a projector,phototransistors 35, 36 as a light receiver, light receiving windows 34a, 34 b, and an amplifier 45 a. The light emitting diodes (LED) 30, 31are preferably infrared light-emitting diodes (IRED) which emit infraredrays since the recording paper 10 is fixative by ultraviolet rays. Thepaper lateral edge detecting sensor 28 as a second sensor consists oflight emitting diodes (LED) 32, 33 as a projector, phototransistors 37,38 as a light receiver, light receiving windows 34 c, 34 d, and anamplifier 45 b.

[0039] The light receiving windows 34 a to 34 d are formed on a headcover 34. LEDs 30, 33 and photo transistors 35, 38 are provided at P1(first positions) on which both lateral edges of the recording paper 10pass. LEDs 31, 32 and phototransistors 36, 37 are provided at P2 (secondpositions). P2 are aligned with P1 on line L2 (see FIG. 2) in parallelwith widthwise direction of the recording paper, and arranged inner ofP1.

[0040] It is possible for the color thermal printer to print theking-sized recording paper 10 and large-sized recording paper 9 (seeFIGS. 2 and 9). The recording paper 10 is larger than the recordingpaper 9 in width. In order to detect the lateral edge position of therecording paper 9 which is narrower than the recording paper 10 inwidth, P2 is adjusted to cross the lateral edges of the recording paper9.

[0041] In FIG. 4, the phototransistors 35 to 38 are attached to acircuit board 39 fixed to the thermal head substrate 22. Note that thephototransistors 35 to 38 may be attached to the ceramic plate providedwith the heat emitting element array 23.

[0042] The inclined head cover 34 is attached to the thermal head 20 soas to direct the recording paper 10 passed through a paper guide cover40 to the heat emitting element array 23. The head cover 34 has afunction of protecting such as a head driver 67 formed on the ceramicplate, in addition to a function of paper guide. The head cover 34covers the phototransistors 35 to 38 and are made of a metallic platesuch as aluminum plate.

[0043] The head cover 34 has the light receiving windows 34 a to 34 dwhich are respectively positioned corresponding to the phototransistors35 to 38 on P1 and P2. The light receiving windows 34 a to 34 d are, forexample, slits crossed at an angle of 45 degree to the lateral edge ofthe recording paper 10 and guides light emitted from LEDs 30 to 33 tothe phototransistors 35 to 38. As shown in FIG. 2, the light receivingwindows 34 a to 34 d are formed linearly symmetric with respect to acenterline (CL) of the transporting path and have enough length to covertraveling deviation of the recording papers 9, 10. The center of thelight receiving windows 34 a to 34 d is found by a mean value of thepaper lateral edges during transport thereof.

[0044] Thus, paper lateral edges are detected by using thephototransistor, so that the printer of the present invention is smallerin size than that of using a CCD line sensor, to shorten the interval(L1) between printing position by heat emitting element array 23 anddetecting position by paper lateral edge detecting sensors 27, 28.Therefore, it is possible to detect the paper lateral edges near theheat emitting element array 23 and to obtain more precise information ofthe lateral edges.

[0045] As shown in FIG. 2, the number of pixels of the images recordedwithin range of a recording area 10 a of the recording paper 10 is, forexample, 1024 in the sub scan direction (L) and 768 in the main scandirection (W). This means that printing for 1024 lines is performed bythe 768 heat emitting elements 24. Note that the actual number of theheat emitting elements 24 on the heat emitting element array 23 is768+α. α is the number of the additional heat emitting elements in orderto extend the length of the heat emitting element array 23 in a mainscan direction longer than the width of the recording paper 10considering transporting deviation and skewing of the recording paper 10in a main scan direction. Practically, α elements in use are severaltens of elements.

[0046] As shown in FIG. 5, in the paper lateral edge detecting sensor 27as a first sensor, output signal from the phototransistors 35,36 fordetecting lateral edge position of the recording paper 10 is divided bya voltage-dividing resistor 46 to be transferred to each terminal of theamplifier 45 a. Since the phototransistor 36 at the second position iscovered with the recording paper 10 as a shielding means, thephototransistor 36 outputs dark current under same environment as thephototransistor 35. The amplifier 45 a amplifies the difference signalbetween the phototransistors 35 and 36. Accordingly, even whentemperature change occurs on the basis of driving of the heat emittingarray, the paper lateral edge senso768 r 27 can obtain output signalcorresponding to positional deviation of the recording paper 10. Thephototransistors 35, 36 are respectively located at a same distance fromthe heat emitting element array 23, thereby to receive the same amountof heat. Accordingly, thermal drift is cancelled by calculating theoutput difference between the phototransistors 35, 36. Also in the paperlateral edge detecting sensor 28 as a second sensor, thermal drift iscancelled in a similar manner to the paper lateral edge detecting sensor27.

[0047] The output signal from the amplifiers 45 a, 45 b are transferredto a system controller 48. Table data is memorized in a memory 49 of thesystem controller 48, and represents shows a relation between outputsignal of the amplifiers 45 a, 45 b and the centerline (PCL) of therecording paper 10. By referring to the table data, the central positiondata of the recording paper is calculated by output signal of theamplifiers 45 a, 45 b. The table data is calculated in advance by usingan actual machine. The heat emitting elements 24 used for printing aredetermined according to the central position data and the paper width.Moreover, pixel lines are associated with each of heat emitting elementsso that a center of the heat emitting elements is in accordance withthat of the image. Furthermore, it is also possible from two lateraledge positions to determine the heat emitting elements 24 used forprinting, by using the data which is for conversion into the lateraledge position on the heat emitting elements 24 from the positionalinformation of the lateral edges position.

[0048] A yellow-fixing lamp 50 and a magenta-fixing lamp 51,constituting an optical fixing device, are disposed downward of thethermal head 20 in a feeding direction. The yellow-fixing lamp 50irradiates near ultraviolet rays, the peak value of which is 420 nm, tofix the yellow thermal coloring layer on the recording paper 10. Themagenta-fixing lamp 51 irradiates near ultraviolet rays, the peak valueof which is 365 nm, to fix the magenta thermal coloring layer on therecording paper 10. A cutter 52 is disposed downstream of the thermalhead 20 in a feeding direction, for cutting the long recording paper 10by each recording area. A printer housing (not shown) has a sheetdischarge opening 53, to discharge a separated print sheet.

[0049] In FIG. 6, the system controller 48 controls each part of thecolor thermal printer. The system controller 48 includes, for example, aCPU, program ROM and the memory 49 such as work RAM. The CPU controlseach part of the color thermal printer according to a control programmemorized in program ROM. Temporary data occurred during control ismemorized in work RAM.

[0050] An IC 57 is connected to the system controller 48. A memorycontroller 55 and an interface controller 56 are packaged in the IC 57.The memory controller 55 controls a memory card 60 loaded in a memorycard slot (not shown) and image memory 61, to read and write the imagedata. The interface controller 56 controls a PC interface 62 and a videooutput circuit 64. The PC interface 62 is used for connecting to apersonal computer and a digital camera, and the video output circuit 64is used for outputting the image to an external monitor 63.

[0051] When displaying the image memorized in the memory card 60 to theexternal monitor 63, the memory controller 55 reads the image data fromthe memory card 60. The image data is transferred to the video outputcircuit 64 by an interface controller 56. The video output circuit 64converts the image data of RGB into a composite signal such as NTSC, totransfer it to the external monitor 63.

[0052] When printing the image memorized in the memory card 60, thememory controller 55 reads the image data from the memory card 60, towrite the image data on the image memory 61. The memory controller 55reads the image data from the image memory 61, to transfer it to a printdata forming circuit 66.

[0053] The print data forming circuit 66 converts the image data (red,green and blue image data) into the print data (yellow, magenta and cyanimage data). Print data having a color to be printed is transferred to ahead driver 67 on a line-by-line basis. The head driver 67 converts theprint data of one line into a driving signal to drive each of the heatemitting elements 24 of the thermal head 20.

[0054] A motor driver 69 and a lamp driver 70 are connected to thesystem controller 48. The motor driver 69 generates a driving pulse fordriving the motor 16 as a stepping motor, by a control signal from thesystem controller 48. The driving pulse generated by the motor driver 69is counted at the system controller 48, to be used for detecting thetransporting amount of the recording paper 10.

[0055] The lamp driver 70 responsive to control signals from the systemcontroller 48 lights on or off the yellow-fixing lamp 50 and themagenta-fixing lamp 51, to fix the yellow thermal coloring layer and themagenta thermal coloring layer.

[0056] The paper lateral edge detecting sensors 27, 28 are controlled bythe system controller 48. In the system controller 48, the lateral edgeposition of the recording paper 10 in a main scan direction is foundbased on the signals from the amplifiers 45 a, 45 b, to determine theheat emitting elements 24 to be driven at the head driver 67.

[0057] Next, the operation of the above embodiment will be explained byreferring to a flowchart in FIG. 7.

[0058] A plurality of thumbnail images stored in the memory card 60 isread by the memory controller 55. Data of the plurality of thumbnailimages is transferred to the monitor 63 via the video output circuit 64in a matrix manner. A user observes the monitor 63 to select the imageto be printed. The memory controller 55 reads the selected image out ofthe memory card 60, to write in the image memory 61.

[0059] When printing is instructed, the system controller 48 controlsthe motor driver 69 and starts rotation of the motor 16. In FIG. 1, themotor 16 rotates the supply roller 12 in the counterclockwise direction.The supply roller 12 rotates the roll 11 to feed the end of therecording paper 10.

[0060] When the end of the recording paper 10 reaches between thecapstan roller 17 and the pinch roller 18, the end-detecting sensor 25transfers the detected signal to the system controller 48. Whenreceiving the detected signal of the end-detecting sensor 25, the systemcontroller 48 starts counting of the driving pulse of the motor 16.According to count of the driving pulse, the transporting amount of therecording paper 10 is measured.

[0061] When it is judged according to the number of the pulses that afirst line 10 b on the recording area 10 a (a hatched portion in FIG. 2)of the recording paper 10 reaches the detected position detected by thepaper lateral edge detecting sensors 27, 28, the controller 48discontinues the rotation of the motor 16, thereby completing thetransport of the recording paper 10.

[0062] The pinch roller 18 is moved by a shift mechanism (not shown)during discontinued transport of the recording paper 10, to pinch therecording paper 10 with the capstan roller 17. The platen roller 21 ismoved by the shift mechanism (not shown) to pinch the recording paper 10with the heat emitting element array 23.

[0063] The system controller 48 lights on the LEDs 30 to 33 of the paperlateral edge detecting sensors 27 and 28, to start detecting the lateraledges of the recording paper 10. The amplifiers 45 a, 45 b calculate thedifference signal between the photo transistors 35 and 36, and betweenthe phototransistors 37 and 38 respectively. The system controller 48determines both lateral edge positions of the first line 10 b in a mainscan direction based on the difference signal between the amplifiers 45a,45 b. The system controller 48 accordingly determines the centerline(PCL) of the recording paper 10 in a main scan direction, to determinethe heat emitting element 24 n corresponding to the centerline (PCL).

[0064] As shown in FIG. 2, the centerline (CL) of the transporting pathis coincident with the centerline (PCL) of the recording paper if thereis no positional deviation of the recording paper 10 in a paper widthdirection. In this case, a center of heat emitting elements on the heatemitting element array 23 is the heat emitting element 24 n whichcorresponds to the centerline (PCL). As shown in FIG. 8, if there ispositional deviation of the recording paper 10 to the side of the paperlateral edge detecting sensor 28 during transport thereof, the heatemitting element 24 n corresponding to the centerline (PCL) alsodeviates to the side of the paper lateral edge detecting sensor 28 fromthe centerline (CL) of the transporting path.

[0065] The system controller 48 transfers positional information of thecenterline (PCL) to the memory controller 55. The memory controller 55reads the image data from the image memory 61 so as to allocate a pixelof the yellow image placed on its centerline in a main scan direction tothe heat emitting element 24 n corresponding to the centerline (PCL) ofthe recording paper 10 in a main scan direction.

[0066] The image data read from the image memory 61 is transferred tothe print data forming circuit 66. The image data of RGB is convertedinto the print data of YMC. After conversion, print data for the firstline of the yellow image is read and transferred to the head driver 67.The head driver 67 converts the print data for the first line into thedriving signal, to drive each of the heat emitting elements 24, therebyto print the first line of the yellow image on the recording paper 10.After printing of the first line of the yellow image is completed, themotor 16 rotates backwards at predetermined steps, to transport therecording paper 10 of one line in a rewinding direction.

[0067] With reference to print of the first line, the system controller48 determines whether each the heat emitting elements should generateheat or not, based on the positional information of both lateral edges,to transfer either drive disabling signal or drive permitting signal,allocated to each the heat emitting elements, to the head driver 67. Thehead driver 67 drives only heat emitting elements permitted to generateheat. Accordingly, there are no heat emitting elements driven outside ofthe lateral edges of the recording paper 10.

[0068] The aforementioned procedure is repeated, so that a second lineof the yellow image is recorded on the recording paper 10. By repeatingprint of one line and transport of the recording paper 10, the yellowimage is printed within the recording area 10 a. Actually, the motor 16is continuously rotated. Simultaneously, the heat emitting element array23 is driven to print one line of the yellow image every transport ofthe recording paper 10 on a line-by-line basis.

[0069] Also in the print of the second line and the following lines ofthe yellow image, both lateral edges of the recording paper 10 aredetected every transport of the recording paper 10 on a line-by-linebasis. Based on positional information of both lateral edges, the heatemitting elements used for printing are determined, and reading theimage data from the image memory 61 is controlled.

[0070] When printing the yellow image is completed, the systemcontroller 48 moves the platen roller 21 to release the pressure of therecording paper 10 to the thermal head 20. Next, the system controller48 starts anti-reverse of the motor 16 to transport the recording paper10 in a feeding direction. The yellow fixing lamp 50 lights on via thelamp driver 70 simultaneous with start of the transport of the recordingpaper 10, to fix the heated yellow thermal coloring layer. When theyellow thermal coloring layer is completely fixed by entirelyirradiating ultraviolet rays to the recording area 10 a, the systemcontroller 48 discontinues the transport of the recording paper 10 tolights off the yellow fixing lamp 50.

[0071] Next, the system controller 48 transports the recording paper 10in a rewinding direction. When the first line lob on the recording are a10 a reaches printing position of the heat emitting element array 23 onthe thermal head 20, the transport of the recording paper isdiscontinued.

[0072] As mentioned above, the platen roller 21 is moved downward topress the recording paper 10 to the thermal head 20. The magenta imageis printed on line-by-line basis during the transport of the recordingpaper 10 in a rewinding direction. Again, lateral edges of the recordingpaper 10 are detected, heat emitting elements used for printing aredetermined, and the centerline (PCL) of the recording paper 10 isadjusted to that of the image.

[0073] When printing of the magenta image is completed, themagenta-fixing lamp 51 lights on, to fix the heated magenta thermalcoloring layer during rewinding of the recording paper 10. Thereafter,printing of the cyan image is also performed in a similar manner, inwhich both lateral edges of the recording paper 10 are detected, heatemitting elements used for printing are determined, and the centerline(PCL) of the recording paper is adjusted to that of the image.

[0074] When printing of the cyan image is completed, the recording paper10 is transported in a feeding direction to be cut into a sheet by thecutter 52. The sheet-shaped recording paper is discharged outside thecolor-heating printer from the sheet discharge opening 53.

[0075] Although the lateral edges are detected each line in thisembodiment, the lateral edges may be detected each several lines or tensof lines. Moreover, lateral edges may be detected only at the start ofrecording each color image if meandering does not occur. The lightreceiving windows 34 b, 34 c arranged inwardly, as a second combinationcorresponding to the phototransistors 36, 37 are shielded by therecording paper 10. However it may be shielded by a shielding sticker.

[0076] The thermal printer can be used for printing on the large-sizedrecording paper 9 in addition to the king-sized recording paper 10. Inthis case, the light receiving windows 34 a, 34 d arranged outwardly asa first combination are covered with light-tight shielding stickers 75as shown in FIG. 9. Accordingly, the paper lateral edge detectingsensors 27, 28 corresponding to the large-sized recording paper 9 becomeeffective.

[0077] Incidentally, the receiving windows may be selected correspondingto the size of the recording paper used for printing, based on thesize-changing signal of the recording papers 9, 10. In FIG. 10 showingthe embodiments, the same reference numerals are used to designate thesame or similar components as those in FIG. 3. The light receivingwindows 34 a, 34 d arranged outwardly are opened and shut by a shieldingshutter 80 which is shifted by a shift mechanism 81. In the large-sizedrecording paper 9 shown in FIG. 10A, the shift mechanism 81 is driven toslide the shielding shutter 80 to the closed position, to shut the firstlight receiving windows 34 a, 34 d. In the king-sized recording paper 10shown in FIG. 10B, the shift mechanism 81 is driven to slide theshielding shutter 80 to the opened position. In printing on theking-sized recording paper 10, the second light receiving windows 34 b,34 c are shielded by the recording paper 10. Note that the shieldingshutter 80 may be moved with swing or rotation.

[0078] In the embodiments shown in FIG. 11, a head cover 85 is made of atransparent material on which two types of mask plates 86, 87 areselectively set. The first mask plate 86 has light receiving windows 86a, 86 b at the position corresponding to the phototransistors 91, 92.The second mask plate 87 has light receiving windows 87 a, 87 b at theposition corresponding to the phototransistors 90, 93. Either of themask plates 86, 87 is selected corresponding to size of the recordingpaper used for printing. The reference numerals 94 to 97 show LEDs. Itis noted that the head cover may be made of opaque material such as ametal, and that openings may be formed to cross the lateral edges of therecording papers 9, 10, and that either of the mask plates 86, 87 may beattached to the head cover so as to close the openings.

[0079] Moreover, it is possible to provide memories for memorizingpositional data of lateral edges of each line. The number of thememories corresponds to distance between the heat emitting element array23 and the paper lateral edge detecting sensors 27, 28. When themeasured lateral edges reach the heat emitting element array, driving ofthe heat emitting element array is controlled based on the correspondingpositional data of the lateral edges. Furthermore, it may be possible todetermine the position of lateral edges at a recording position based onthe skew running of the recording paper based on the positional data ofthe lateral edges of each line, and distance between the heat emittingelement array and the edge detecting sensors.

[0080] It may be possible to measure reflected light from the lightreceiving windows 34 a to 34 d in place of transmitted light. In thiscase, LEDs are provided on the side of phototransistors 35 to 38, to beprojected toward the light receiving windows 34 a to 34 d.

[0081] Moreover, both lateral edges of the recording paper are detectedaccording to the above-mentioned embodiments. However, only one lateraledge of the recording paper may be measured since the width of therecording paper is constant. In this case, it is possible to knowposition of the other lateral edge by adding paper width to position ofone lateral edge. If one of the lateral edges is moved along a guideplate, it is sufficient to only search a position of the other lateraledge.

[0082] Three or more types of the recording papers may be used. In thiscase, light receiving windows, LEDs, and phototransistors are arrangedat the position corresponding to the lateral edges of each type of therecording paper. An unrequired light-receiving window is shielded with alight-tight shielding sticker or a shutter.

[0083] The present invention is applicable not only to a color thermalprinter of a one-head, three-pass type but also that of a three-head,one-pass type. Moreover, the present invention can be used for such ascolor thermal transfer printer, a color ink-jet printer, a laserprinter, a copying machine, and other image forming apparatuses ofvarious kinds. Furthermore, the present invention can be applicable to aweb-conveying device. The recording head may be moved relative to therecording paper in stopping, in which the lateral edge detector is movedwith the recording head.

[0084] Although the present invention has been fully described by theway of the preferred embodiments thereof with reference to theaccompanying drawings, various changes and modifications will beapparent to those having skill in this field. Therefore, unlessotherwise these changes and modifications depart from the scope of thepresent invention, they should be construed as included therein.

What is claimed is:
 1. A sheet lateral edge detector comprising: a lightreceiving window having a region which a lateral edge of a recordingsheet moves across; a first photo sensor for measuring light passingthrough said light-receiving window partially shielded by said lateraledge of said recording sheet; a second photo sensor disposed away fromthe first photo sensor in a widthwise direction of said recording sheet,for measuring dark current; and a judging device for judging theposition of said lateral edge of said recording sheet from thedifference signal between output signals of the first and second photosensors.
 2. A sheet lateral edge detector as claimed in claim 1, furthercomprising a recording head, for performing line print of the image onsaid recording sheet, wherein said first and second photo sensors areattached to said recording head, wherein said recording sheet is movedrelative to said recording head.
 3. A sheet lateral edge detector asclaimed in claim 2, further comprising a head cover slantly attached tosaid recording head, for guiding said recording sheet, wherein saidlight receiving window is formed on said head cover.
 4. A sheet lateraledge detector for a printer, said printer being capable of printing onfirst and second recording sheets, a width of said first recording sheetbeing larger than a width of said second recording sheet, said sheetlateral edge detector comprising: a first light receiving window havinga region while a lateral edge of said first recording sheet moves acrossduring transport thereof; a first photo sensor for receiving lightpassed through said first light receiving window partially shielded bysaid lateral edge of said first recording sheet; a second lightreceiving window having a region which a lateral edge of said secondrecording sheet moves across during trasnport thereof; a second photosensor for receiving light passed through said second light receivingwindow partially shielded by said lateral edge of said second recordingsheet; a recording head for printing on said first or second recordingsheet on line-by-line basis extending in said widthwise direction, saidrecording head having a length larger than said width of said firstrecording sheet; a shielding device for shielding said second lightreceiving window in printing on said first recording sheet, and forshielding said first light receiving window in printing on said secondreceiving sheet; and a judging device for judging the position of saidlateral edge of said first or second recording sheet from the differencesignal between output signals of the first and second photo sensors. 5.A sheet lateral edge detector as claimed in claim 4, wherein saidrecording head is a thermal head on which a plurality of heat emittingelements are aligned.
 6. A sheet lateral edge detector as claimed inclaim 5, wherein said first and second photo sensors are aligned nearsaid recording head and in parallel therewith.
 7. A sheet lateral edgedetector as claimed in claim 6, wherein said first and second lightreceiving windows are in a slit form inclined with respect to adirection of said transport of said first or second recording sheet. 8.A sheet lateral edge detector as claimed in claim 7, further including ahead cover, secured by said thermal head, for guiding said first andsecond recording sheets, said first and second light receiving windowsbeing formed on said head cover.
 9. A sheet lateral edge detector asclaimed in claim 8, wherein said first and second photo sensors areattached to said thermal head.
 10. A sheet lateral edge detector asclaimed in claim 9, wherein said first recording sheet is functioned assaid shielding device in printing on said first recording sheet.
 11. Asheet lateral edge detector as claimed in claim 10, further comprising ashielding shutter for opening and shutting said first light receivingwindow, functioned as said shielding device in printing on said secondrecording sheet.
 12. A sheet lateral edge detector as claimed in claim7, wherein said shielding device includes a first mask plate providedwith said first light receiving window and a second mask plate providedwith said second light receiving window, wherein said first mask plateis set between said first recording sheet and said first and secondsensors in printing on said first recording sheet, wherein said secondmask plate is set between said second recording sheet and said first andsecond sensors in printing on said second recording sheet.
 13. A sheetlateral edge detector as claimed in claim 12, further comprising a headcover secured to said thermal head, for guiding said first and secondrecording sheets, said head cover having a transparent part forselectively setting said first and second mask plates.